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Bulawayo: Balancing health and environmental needs
Fungai Sexton Ndawana Makoni
Thesis submitted in fulfilment of the requirements for the degree
Philosophiae Doctor in the Faculty of Natural and Agricultural
Sciences, Department of Zoology and Entomology, University of
the Free State
March 2014
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University of the Free State Qwaqwa Campus, Private Bag X13,Phuthaditjhaba
9866, South Africa
Prof. P. A Mbati, Principal and Vice-Chancellor, University of Venda Private Bag
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original work and that I have not previously in its entirety or in part submitted it at any university for a degree. I furthermore, cede copyright of the thesis in favour of the University of the Free State
25 March 2014
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To my late father Tirivanhu, Gwenzi Ndawana Makoni, A renounced educationist who passed away on 31 August 2013, in the final stages of the preparation of this thesis.
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First and foremost I would like to thank my supervisors Professor. P. A Mbati and Dr. O. M. M. Thekisoe who kept on urging me when the going seemed to have come to an end. My special thanks go to Professor Ogola and Professor Odiyo from the University of Venda for their unwavering support and for accommodating me in the school of Environmental Science, especially the in Department of Hydrology and Water resources.
I would like to thank my colleagues at the Institute of Water and Sanitation Development for the encouragement and support, the late Mrs Neseni, Mr Gabriel Chishiri and Dr. Lawrence Nyagwambo.
My appreciation also goes to the farmers with whom I worked with during the data collection exercise. My thanks also go to Mr. Irvine Ncube, (Bulawayo City) and Messers Sherman Mutengu, Emmanuel Machache and Tafadzwa Mahere.
I also want to express my gratitude to Waternet for the support rendered to finalise the thesis. I am happy that this has come to the end, but I would love to pay tribute to the man whom I would have loved to see this product to finality. To the Late Dr. Jerry Ndamba – here is what you all started! thank you.
Finally I would like to thank my family especially my wife Idah and the children for their patience and encouragement through this study. I would like to dedicate this work to my twin daughters, maybe for understanding or not understanding why their father did not give them the required attention when they needed it most. Lastly, i would like to thank my parents, sisters and brothers who also kept encouraging me. I thank you all.
vi DECLARATION iii ACKNOWLEDGEMENTS v LIST OF ABBREVIATIONS ix DEFINITION OF TERMS x SUMMARY xv CHAPTER 1 2
Introduction and Literature review 2
1.0 Urban wastewater use in agriculture 2
1.1 Urban agriculture 4
1.2 Benefits of urban agriculture 6
1.3 Urban agriculture irrigation with wastewater 7
CHAPTER 2 9
2.1 Specific research problem 9
2.2 Research Objectives 9
2.2.1 General Objective 9
2.2.2 Specific objectives 10
2.3 Research scope 10
2.4 Relevance of the research 10
CHAPTER 3 12
Determination of physical, chemical and biological characteristics of treated domestic
effluent used for urban agriculture irrigation in Bulawayo 12
3.1 Introduction 12
3.2 Municipal wastewater treatment systems 13
3.3 Biological nutrient removal system (BNR) 14
3.4 Waste stabilisation ponds 15
3.5 Nitrogen and phosphorus 16
3.6 pH 16
3.7 Metals in wastewater 17
3.8 Lead 18
3.9 Materials and Methods 18
3.10 Study area 18
3.11 Approaches in data collection and analysis 20
3.12 Sampling procedure 20
3.13 Effluent sample collection 21
3.14 Field based measurements 21
3.15 Faecal and total coli form 22
3.16 Laboratory based analysis 23
3.17 Results 23
3.17.1Effluent characteristics of treated domestic wastewater used in Bulawayo 23
3.17.2Effluent temperature 23
3.17.3Effluent pH 24
3.17.4Effluent turbidity 24
3.17.5Electrical conductivity (EC) 25
3.18 Effluent chemical parameters 26
3.18.1Effluent total nitrogen 26
3.18.2Effluent total phosphorus 26
3.18.3Cadmium concentration of the effluent 26
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3.20.3Effluent turbidity 30
3.20.4Electrical conductivity 30
3.20.5Effluent cadmium and lead 31
3.21 Conclusions 32
Chapter 4 34
Chemical and heavy metal content in soils and crops under irrigation with treated
domestic wastewater effluent in Bulawayo 34
4.1 Introduction 34
4.2 Cadmium 35
4.3 Lead 37
4.4 Study area 38
4.4.1 History of the Luveve farming area 39
4.4.2 Major crops grown at the plantation 39
4.4.3 Source of the irrigation water 40
4.5 Materials and methods 40
4.5.1 Sampling procedure 41
4.5.2 Soil analysis 42
4.6 Vegetable tissue metal analysis 42
4.8 Results 43
4.8.1 Soil pH 43
4.8.2 Soil nitrogen 44
4.8.3 Soil Texture 44
4.8.4 Soil phosphorus 45
4.8.5 Sodium adsorption ratio (SAR) of the soil 45
4.8.6 Soil cadmium 45
4.8.7 Lead in soil 47
4.8.8 Pollution load index 48
4.8.9 Vegetable Tissue metal analysis 49
4.9 Discussion 49
4.10 Conclusion 51
Chapter 5 53
Assessment of the potential socio-economic benefits of wastewater use in small scale
urban agriculture irrigation in Bulawayo. 53
5.1 Introduction 53
5.2 Urban agriculture (AU) 56
5.3 Production systems, input use and outputs 57
5.4 Post production and marketing activities 58
5.5 Methodology 59
5.6 Results 60
5.6.1 Wastewater users 60
5.6.2 Socio-economic benefits 62
5.6.3 Fertiliser contribution from the wastewater 62
5.7 Discussion 63
5.8 Conclusion 64
Chapter 6 65
Proposed management strategy for sustainable utilisation of treated domestic wastewater
for irrigated agriculture in Bulawayo 65
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6.2.3 Implications for farmer use 69
6.2.4 Support research and outreach 69
6.2.5 Marketing 70
6.2.6 Stakeholder forums and participation. 71
6.3 Conclusion and recommendations 72
Chapter 7 73
7.1 General discussion and conclusions 73
7.2 Recommendations 75
ix ANOVA: Analysis Of Variance
BCC: Bulawayo City Council
BNR: Biological Nutrient Removal
BOD: Biological Oxygen Demand
Cd: Cadmium
CEC: Cation Exchange Capacity
DWAF: Department of Water Affairs and Forestry E.coli: Escherichia coli
EC: Electrical conductivity
FAO: Food and Agriculture Organisation of the United Nations
Hg: Mercury
IDRC: International Development Research Centre IMWI: International Water Management Institute IWRM: Integrated Water Resources Management LTC: Long Term recommended Concentration
MDGs: Millennium Development Goals
NGO: Non- Governmental Organisation
Pb: Lead
PE: Person Equivalent
PLWHA: People Living With HIV and AIDS
RUAF: Resources for Urban Agriculture Foundation
SAR: Sodium Absorption Ratio
STC: Short Term recommended Concentration TARWR: Total Actual Renewable Water Resources TDS: Total Dissolved Salts
UA: Urban Agriculture
UN: United Nations
UNDP: United Nation Development Programme
UNESCO: United Nations Educational Scientific and Cultural Organisation
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In the thesis the following working definitions have been adopted:
Cost recovery Cost recovery refers to the process of setting a tariff which ensures that capital and/or recurrent costs are partially or fully covered, billing and ensuring that all users pay their bills on time.
Environmental sanitation
This covers the concept of controlling all factors in the physical environment which may have deleterious impacts on human health and well-being. It includes clean and pathogen free environments and treatment and safe disposal of human excreta, storm and wastewater and solid waste.
Food poverty line
The amount of income required to buy a basket of basic food needs for one person per year.
Household Household refers to an entity that takes and acts upon decisions about consumption and investment. In this thesis the term households is used interchangeably with “individual” or “consumer” depending on the context.
Informal settlements
Poor urban settlements such as slums, shantytowns and peri-urban areas. These areas are characterised by high population densities, poor housing, sewerage and drainage facilities, few or no paved streets, irregular waste clearance, low income and professional diversity, mainly unskilled in nature. In this thesis the words “informal settlement”, “poor urban areas”, “squatter settlement” and “slum” are used interchangeably.
Institutions The rules and regulations that govern the relationships between organisations, the standard of services and the way services are provided.
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sanitation
Sanitation systems which are contained within the plot occupied by the dwelling. On-plot sanitation is associated with household latrines.
On-site sanitation
Includes communal facilities which are self-contained within the site (pit latrines for example), in contrast to sewerage and dry latrines where excreta is removed from the site.
Agricultural irrigation use
According to IWMI (2000,b), agricultural irrigation use refers to the use of water for the purposes of planting, cultivating and harvesting of agricultural products, and for processing, particularly where the products have not been subjected to any agro-industrial processing.
Sewage Wastewater that usually includes excreta and that is, will be, or has been carried in a sewer.
Sewerage System of interconnected pipes or conduits through which sewage is carried.
Tenure A bundle of rights, which regulate access, use and ownership over land and other resources (Land for example).
Water Resources:
Domestic wastewater
In this thesis refers to all water available for human use, namely domestic use, agriculture and industry.
Refers to effluent consisting of black water (excreta, urine and faecal sludge, i.e. toilet wastewater) and grey water (kitchen and bathing wastewater).
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Water Scarcity In this thesis is refers to when an individual does not have access to safe and affordable water to satisfy her or his needs for drinking, washing or their livelihoods(Rijsberman, 2004).
Wastewater recycling
This is defined by Pescod, (1992) as the planned and deliberate use of treated wastewater for some beneficial purposes, such as irrigation, recreation, industry, the recharging of ground aquifers and drinking water
Unrestricted irrigation
IWMI (2000,b) defines unrestricted irrigation as the unlimited use of wastewater for purposes of planting, cultivating and harvesting of agricultural products such as forage, grains, fruits, vegetables and greens.
Restricted irrigation
This is the use of wastewater for purposes of planting, cultivating and harvesting of agricultural products, except vegetables and greens which are consumed raw (Looker, 1998).
Heavy Metals Heavy metals is a collective name given to all metals above calcium in the Periodic Table of Elements, which can be highly toxic, and which have densities greater that 5g/cm3. The main heavy metals of concern in freshwater include lead, copper, zinc, chromium, mercury, cadmium and arsenic.
Urban poor These are people who live in informal settlements of the urban areas and earn incomes which are below the total consumption poverty datum line.
Urbanisation The process by which an increasing proportion of the population comes to live in urban areas. Urbanisation also includes the process which causes this change which is usually a combination of economic, social and political change.
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Figure 1: Google map of Bulawayo showing wastewater treatment plants
in relation to the Luveve gum plantation 19
Figure 2: Effluent sample collection 21
Figure 3: Mean effluent temperature 24
Figure 4: Mean EC values against the minimum recommended standard 25
Figure 5: Total phosphorous effluent concentration 26
Figure 6: Mean Cd effluent concentration 27
Figure 7: Lead concentration in the effluent 28
Figure 8: Showing of the major crops grown maize and chomolier 39
Figure 9: Showing canal delivery wastewater to plantations and earth
canal in the fields 40
Figure 10: Soil pH in the irrigated and control soils and the three depths 44
Figure 11: Correlation of soil pH and Cd concentration 46
Figure 12: Mean Cd concentration in soils 46
Figure 13: Correlation of Soil pH and Pb concentration 47
Figure 14: Average lead soil concentration with depth 48
Figure 15: Projected water scarcity in 2025 (Adapted form World Water Council,
2009) 55
Figure 16: Picture showing maize produce (a) and farmers in their field (b) 61
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Table 1: Questions about urban agriculture 7
Table 2: Zimbabwe concentration of trace elements in wastewater suitable for
irrigation 17
Table 3: Parameters measured and methods used 20
Table 4: Guidelines for EC and TDS discharge into surface waters 31
Table 5: Relative accumulation of Cadmium into edible parts of different crops 36
Table 6: Threshold levels of some selected trace elements for crop production in
wastewater (Source: Metcalf and Eddy 1991) 37
Table 7: Parameter measured and laboratory used 41
Table 8: Field Soil Texture 44
Table 9: Vegetable tissue metal analysis 49
Table 10: Showing positive and negative perception of stakeholders ( policy makers
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The use of urban wastewater for agriculture crop production is receiving renewed attention in most parts of the world due to the increasing scarcity of water. Water scarcity has placed pressure on the ability of households to meet their basic needs as the intermittent supply of water has created a demand for other sources of water, such as wastewater for irrigation, which can either be expensive or dangerous to public health. In this regard it might seem obvious to view wastewater as a major source of water for Bulawayo, Zimbabwe, particularly for irrigation.
The general objective of the study was to characterise and determine treated domestic wastewater parameters that are of agricultural, public health and environmental importance for use in urban agriculture irrigation in Bulawayo. The study critically assessed the wastewater quality being used for urban agriculture in relation to its characteristics, the possible impacts on environment health and also quantifying the socio economic factors that can be derived from its use and, based on this assessment, to formulate a strategy for sustainable treated domestic wastewater use for irrigation. Data collection for this study was conducted in Bulawayo urban area and the gum plantation from 2005 - 2010.
Extensive wastewater quality analysis was carried out and results of effluent analysis of key parameters, nitrogen and phosphorous were found to be 11.5 mg/l ± 4.4 and 13.5 mg/l ±14.9 respectively, which were within the World Health Organisation (WHO) (2006) acceptable range. These results aided to confirm that the treated domestic wastewater is of acceptable quality and hence has potential to be used for irrigating crops such as maize, beans and vegetables (chomolier) with minimal risk. Effluent heavy metals concentration in the form of Cadmium (Cd) and lead (Pb) measured values of 0.027 mg/l ± 0.01 and 0.45 mg/l respectively and were within the acceptable levels according to the WHO guidelines and Zimbabwe National Water Authority (ZINWA) standards.
Heavy metal soil content was also observed to be within the acceptable limits with both Cd and Pb showing strong correlation with soil pH (r2= 1). Vegetable tissue
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which then confirmed the conclusion that the treated domestic wastewater has potential for agricultural irrigation provided the quality of the effluent would not change drastically from the observed status which was measured over five years.
With regard to social acceptance and economic benefits, the study revealed that acceptance for use of treated domestic wastewater and consumption of produce from its use was high amongst the farmers with 88.9% of respondents acknowledging no problem in using the treated domestic wastewater. Estimation of financial benefits were derived using the conventional market based approach which then revealed that an income of about US$1000 per plot/year is feasible if a proper management system is put in place. Findings of this study confirm that use of treated domestic wastewater for urban irrigation can improve livelihoods of the resource limited farmers despite the health challenges associated with its use. Majority of the famers reported that use of treated domestic wastewater for agriculture has contributed significantly to their socio- economic lifestyles by making extra income to cover school fees (44.6%), medication (9.85%) and food (99.1%). Apart from the financial benefits observed, calculations using the FAO formula for nutrient contribution, the study indicated that the treated domestic wastewater which was used contributed approximately 92 kg/ha/year, 108 kg/ha/year and 281.6 kg/ha/year of nitrogen, phosphorus and potassium respectively hence improving soil fertility of the sandy loam soils found at the farming area.
Evaluation of the findings in relation to the recommended guidelines and standards of Food and Agricultural Organisation (FAO)/WHO and ZINWA suggests that the treated domestic wastewater used at the gum plantation is suitable for crop irrigation specifically for the following crops: chomolier, maize and beans that were investigated over time. In addition the benefits of using the treated domestic wastewater were noted to have the potential to enhance proper management of wastewater irrigation as proposed in the strategy as it proved to be a reliable water resource. Adherence to the strategy that is proposed in this thesis of involving stakeholders, addressing policy and legal issues, supporting research and outreach,
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Key words: Wastewater use; environment, public health, effluent, agriculture, water resource, livelihoods
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CHAPTER 1
Introduction and Literature review
1.0 Urban wastewater use in agriculture
Urban environmental management has become critical as the urbanisation of developing nations continues on an upward trend. This urbanisation has introduced many challenges to urban planners and managers among them being the need to ensure that basic human services are maintained in proportion to the population, such as water, sanitation and the management of wastewater. Poor management of industrial and domestic wastewater in many urban areas in most developing countries is a major problem and this contributes to the contamination of locally available fresh water supplies with a degenerative effect on public health and the environment (WHO, 2010).
Use of urban wastewater for agricultural crop production is receiving renewed attention in most parts of the world due to the increasing scarcity of fresh water resources in many arid and semi-arid regions. The need for clean household water supplies continues to increase and often competes with the need for agricultural water. It has been estimated that many countries the world over especially in sub-Saharan Africa are entering a period of water shortages and this has created competition amongst sectors such as agricultural irrigation which has resulted in severe pressure for the resource as irrigation is by far the largest user of water accounting for over 70% of all water uses (International Water Management Institute (IWMI, 2006). Agricultural irrigation represents a significant fraction of the total demand for fresh water (Hamilton, et al., 2007). Water shortages are becoming an increasingly noticeable problem in many regions of the world, especially in Sub- Saharan Africa (IWMI, 2000(b); Turton, et al., 2003; Taigbenu and Ncube 2005, Scheierling, et al., 2010).
The use of treated domestic wastewater for agricultural purposes has been demonstrated to promote the concept of recovery and use of nutrients. This concept has been shown to be particularly attractive since this allows for the expansion of
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intensive agriculture while preserving limited resources of good quality water for the rapid urban development that is taking place in many developing nations (Feachem,
et al., 1977; Mazari- Hariat, et al., 2008).
In the developing world very little focus has been made in the past to wastewater treatment for reuse, however water treatment has received more priority than wastewater collection and treatment. According to Ensink, et al., (2002), wastewater reuse is believed to be a low cost alternative to conventional irrigational water though it may carry and cause health and environmental risk due to its high contents of heavy metals and faecal transmitted pathogens. Despite the risks wastewater has, it has been demonstrated in other regions that if used effectively it will become an important resource for urban agriculture (Mazari- Hariat, et al., 2008; SIWI and IWMI, 2004).
Concern for public health has been the major constraint in the use of wastewater for crop production since it carries a wide spectrum of pathogenic organisms posing risk to agricultural workers, crop handlers and consumers (Blumenthal, et al., 2000 and Maldonado, et al., 2008). Ensink, et al., (2002) pointed out that in using wastewater for irrigation, the direct health risks are localized within an irrigated area and the exposed group is relatively small. Looker (1998) noted that the greatest challenge in the water and sanitation sector over the next decades will be the implementation of a low cost sewage treatment system that will at the same time permit selective reuse of treated effluent for irrigation and industrial purposes, taking into account public health concerns.
The use of wastewater in agriculture crop production has been demonstrated to have many benefits among them conservation of and more rational allocation of fresh water resources, recovery of nutrients which will result in reduced requirements for artificial fertilizers, reduction of pollution loads to receiving waters and can provide farmers with a reliable water supply. Thus urban wastewater management should be viewed not only from a disposal based linear system but from a recovery based closed loop system. Studies in India have proved that wastewater use for crop production
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increased yield by 30 - 40% (Pescod and Arar, 1985), and studies by IDRC 2004, indicate that harvests from wastewater have obtained income of US$340/ha in Nairobi, Kenya.
1.1 Urban agriculture
In this millennium the most significant developmental challenges are, rapid urbanization and growing poverty. These trends are more visible in cities and towns in Africa and Asia, where population growth will almost treble from 414 million now to more than 1.2 billion by 2050, while Asia will grow from 1.9 billion to 3.3 billion in that period (United Nations [UN], 2013) This implies that more than half of the population of Africa and Asia will live in urban areas. This development will consequently create pressure on most municipalities and governments to provide adequate infrastructure for social services, which most African municipalities are struggling with. Due to the difficulties in providing social services as well as creating income generating opportunities, most of Africa’s urban population will resort to self-help activities in a bid to satisfy their basic households needs especially food (Argenti, 2000; Argenti, 2001).
Food security is one of the basic needs and as a result urban agriculture, both legal and illegal, has grown as a consequence of the difficult economic environment which most African countries are experiencing. Urban agriculture in some way has been seen to provide the lifeline, as an important means to supplement food supplies as well as household income. However urban farming has often been regarded as being merely “kitchen gardening” or marginalised as a leftover of rural habits. Various authors have defined, urban and peri-urban agriculture as the growing of plants and raising of animals for food and other uses which involve the production, delivery of inputs, the processing and marketing of products within cities and peri-urban areas (Mouget ,1999; Nugent, 2000; IDRC, 2003(a)).
Urban Agriculture has become part of food security system in the urban areas of most countries in Eastern and Southern Africa. It has expanded massively in the last twenty years in response to changes in the micro-economic environment characterized by poor economic performance resulting in increased poverty levels in the urban areas (UNDP, 2006; Bakker, et al., 2000; IDRC, 2003(a); RUAF, 2003). It
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is in the past decades that local authorities and central governments have recognized urban agriculture as a legitimate land use in some countries, Zimbabwe included. It is now generally recognized that urban and peri-urban agriculture contribute to household food security and also has a wide role in sustaining urban population in terms of poverty alleviation and contribution to the urban economic activities, through processing and marketing of the produce (IDRC, 2003(b); RUAF, 2003). Most governments and local authorities now support urban agriculture and are seeking for ways in which to facilitate sustainable, safe and profitable production. In addition, urban agriculture is now an established strategy for sustaining livelihoods of urban populations, as it has been shown to directly provide food and indirectly generate household cash income, through saving on food expenditure, employment and selling of surplus production.
Urban and peri-urban agriculture have been incorporated into urban expansion plans of Dar-es-Salaam, Dodoma in Tanzania and Maputo in Mozambique (Mouget, 1999). Active programmes exist in most cities in Southern Africa. In Zimbabwe, several cities and municipalities now have an accommodating approach to urban agriculture. The Ministry of Local Government and National Housing has pledged more land from acquired surrounding farms to local urban authorities for urban agriculture.
Several studies have been conducted and studies by Mbiba (1995); Mudimu (1996); and Nuwagaba and Atukunda (2001) show that urban and peri-urban agriculture contributes greatly to the food security of many urban residents. Urban agriculture enhances considerably the degree of self-sufficiency in cereal, fresh vegetable and small livestock production. It also affords savings that can be spent on non-produced foodstuff or other needs and generates principal income that can be reinvested in other urban businesses (Mouget, 1999). Urban and peri-urban agriculture also provide employment to a large number of urban residents. In Nairobi, Kenya, for example, 25% of the population is employed in urban and peri-urban agricultural activities (Nugent, 2000).
Urban and peri-urban agriculture varies from city to city and country to country. In Bulawayo (Zimbabwe) the main forms of urban agriculture are off-plot and on-plot. Off-plot cultivation and livestock grazing take place along railway lines, open areas,
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on the periphery of parks, undeveloped public and private land, properties of schools and churches and urban fringe. A review of existing bibliography on urban agriculture experiences in Zimbabwe demonstrate that most urban agriculture activities are in the form of family vegetable gardens and small scale pieces of land. The main produce comprises of vegetables (tomatoes, squash, beans, lettuce, onions, maize among others) with a mixture of small livestock (pigs, chickens, hens, rabbits, etc.), which are fed with the vegetal product residues (Mudimu, 1996).
1.2 Benefits of urban agriculture
United Nations Development Programme (UNDP, 2006) estimated that about 15% of food production in the world comes from urban agriculture (farming, horticulture, animal husbandry, fish ponds, etc.). Nearly 1 billion people are engaged in urban agriculture, 200 million producing food for markets (UNDP, 2006; Bakker, et al., 2000; IDRC, 2003(b); RUAF, 2003). In cities such as Lusaka and Dar-es-Salaam, as much as 50% of the food is produced within the city. Shanghai, which has a population of 11 million, produces 100% of its fresh vegetables in community gardens (Yi-Zhang, 2000).It has been estimated that having market gardens located throughout suburbs and cities could cut the dollar cost of food by 70%. Given that half of the world population soon will live in urban areas, it could be expected that re-circulation of nutrients in urban areas will be featured high on the agenda in the planning for urban agriculture (IDRC, 2004). Despite many benefits, urban agriculture has shown it is still an ill-understood industry (Mbiba 1995; Bakker, et al., 2000) with many questions arising from its practice and some of the questions are shown in table 1.
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Table 1: Questions about Urban Agriculture. Questions about Urban Agriculture: adapted from Gumbo 2005
Where are urban agricultural activities concentrated and why? Who is involved?
Why doing Urban Agriculture (AU)? Is it for psychological or cultural reasons?
What kinds of crops are grown and by which groups of city dwellers? What contribution does the product make to nutrition and food
security?
What type of land tenure system has to be adopted to ensure sustainability?
How available is water and what is its quality? What are the risks to human health?
What are the possible environmental impacts from urban agriculture? How can harmful health and environmental impacts be mitigated? What are the possibilities and limitations for integration of urban
agriculture in urban planning and zoning?
1.3 Urban agriculture irrigation with wastewater
Urban agriculture irrigation with municipal wastewater is practiced in many urban and peri-urban areas of developing countries. In Zimbabwe wastewater irrigation has been practiced for over 30 years as a means of diverting effluent and sludge that does not meet standards for disposal into the natural courses. The practice has largely been restricted to pasture irrigation (Chimbari, et al., 2003). In Windhoek municipal wastewater has largely been used to irrigate sports fields and golf courses (City of Windhoek, 1996).
Wastewater has been demonstrated to be a cheaper and more reliable water resource for agriculture in low-income dry areas (WHO, 2006). Wastewater contains nitrogen and phosphorus which might result in higher yields compared to freshwater irrigation without additional fertilizer application. It was also demonstrated that the
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cost of using wastewater was cheaper than canal water irrigation, although wastewater farmers required more frequent and intensive labour inputs (Scott, et al., 2004; Scott, et al., 2005).
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CHAPTER 2
2.1 Specific research problem
The success of using wastewater for crop production largely depends on adopting appropriate strategies for optimizing crop yields and quality, while maintaining soil productivity, public health safety and safe guarding the environment. However, in planning wastewater use, high priority must be given to the public health considerations since wastewater carries a potentially dangerous load of pathogenic micro-organisms and chemical poisonous contaminants that can be infectious to man. Health criteria must be established to ensure that the benefits by additional water resources are not neglected by unreasonable public health risks to the workers and the public at large (Shuval, 1977; Shuval, et al., 1986; WHO, 2010). Health considerations are centred on wastewater quality, particularly in relation to the irrigation of health sensitive crops which include fruits and vegetables, which are sometimes eaten uncooked (Pescod, 1992; WHO, 2010).
The need to successfully use treated domestic wastewater for crop production motivated this study to characterise treated domestic wastewater in Bulawayo City to aid in the adoption of appropriate strategies of optimizing crop yields and making wastewater use safer and more sustainable. From available literature it has been noted that strict standards may not be sustainable and may lead to reduced health protection since it will be viewed as unachievable under local conditions and thus risk being not adhered to (WHO, 2006; WHO, 2010).
2.2 Research Objectives
2.2.1 General Objective
To characterise and determine wastewater parameters that are of agricultural, public health and environmental importance for use in urban agriculture irrigation in Bulawayo.
10 2.2.2 Specific objectives
1. To determine the physical, chemical and biological characteristics of the
treated domestic effluent used for urban agriculture irrigation in Bulawayo.
2. To assess the levels of lead (Pb) and cadmium (Cd) in soils and crops under
irrigation with treated domestic wastewater effluent in Bulawayo.
3. Assess the suitability of the effluent used in comparison to existing guidelines
and standards for urban agriculture irrigation.
4. To quantify potential socio-economic benefits of wastewater use in small
scale urban agriculture irrigation in Bulawayo.
5. To propose a management strategy for sustainable utilisation of treated
domestic wastewater for irrigated agriculture in Bulawayo.
2.3 Research scope
The research focused on two aspects. The first being collection of information on wastewater quality with regard to physical, chemical and biological parameters. This was used to validate and assess the suitability of the effluent used in relation to guidelines and standards for crop production and environmental management. The cowdry park gum plantation was taken as a reference point for the development of sustainable measures for wastewater use for crop production. The second aspect dealt with collection of information on potential socio- economic benefits of using wastewater which was used to formulate a strategy for use in Bulawayo and similar towns.
2.4 Relevance of the research
Use of urban wastewater for agriculture crop production is receiving renewed attention in most parts of the world due to the increasing scarcity of fresh water resources in many arid and semi-arid regions. In the developing world, including Zimbabwe, limited focus has been made in the past on wastewater treatment for
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reuse, however water treatment received more priority than wastewater collection and treatment. According to Ensink, et al., (2002) and WHO (2010), wastewater reuse is believed to be a low cost alternative to conventional irrigational water though it may carry and cause health and environmental risk due to high contents of heavy metals and faecally transmitted pathogens. Despite the risks wastewater has, it has been demonstrated in other regions that if used effectively it will become an important resource for urban agriculture (Mazari- Hariat, et al., 2008; SIWI and IWMI, 2004; WHO, 2010). Thus it should be acknowledged that conditions vary in many regions of the world and hence an understanding of the context is critical to derive maximum benefits as opposed to a straight adoption of findings from other areas which might not necessarily be applicable to our situation.
The use of wastewater in crop production has also been demonstrated to have many benefits. These include conservation of and more rational allocation of fresh water resources, recovery of nutrients, which will result in reduced requirements for artificial fertilizers, reduction of pollution loads to receiving waters and provision of a reliable water supply to farmers. Thus, urban wastewater management should be viewed not only from a disposal based linear system but from a recovery based closed loop system.
This research took into account the above considerations and will contribute to:
i. The provision of data collected over a long time to determine the status of the treated domestic wastewater used for agriculture and its suitability for sustained urban agriculture in Bulawayo in relation to public health and environmental health.
ii. A better understanding of the problems in terms of the quality of the treated domestic wastewater particularly heavy metals accumulation in soils and crops irrigated by domestic wastewater.
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CHAPTER 3
Determination of physical, chemical and biological characteristics of treated domestic effluent used for urban agriculture irrigation in Bulawayo
3.1 Introduction
The characteristics of the wastewater discharges will vary from location to location depending on the population and the industrial sector served, land uses, groundwater levels, and degree of separation between storm water and sanitary wastes. Domestic wastewater includes typical wastes from the kitchen, bathroom, and laundry, as well as any other wastes that people may accidentally or intentionally pour down the drain. Sanitary wastewater consists of domestic wastewater as well as those discharged from commercial, institutional, and similar facilities (Rose, 1999). The range of flow usually varies from a minimum of about 20% to a maximum of about 400% of the average dry weather flow for small communities and about 200% for larger communities. Industrial wastes will be as varied as the industries that generate the wastes. The quantities of storm water that combines with the domestic wastewater will vary with the degree of separation that exists between the storm sewers and the sanitary sewers. Most new sewerage systems are separate, collect sanitary wastewater and storm wastes, whereas older combined systems collect both sanitary wastewater and storm water (The Green Lane, 2002).
The rate of wastewater generation is usually in the range 80-200 litres per person per day, or 30-70 m3 per person per year (Mara & Cairncross, 1989; Sakhel, et al., 2013). Thus in semi-arid areas with a water demand of 2m3 per year, one person’s wastewater could be used to irrigate 15-35 m2 of land. Municipal wastewater consists mainly of a mixture of water and waste which generally includes dissolved solids and suspended materials made up human and animal wastes, soaps, oils, greases, vegetable and animal residues, household chemicals, soil, bacteria and viruses (Kandiah, 1994a &1994b; WHO, 2010).
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Chemically, wastewater is composed of organic and inorganic compounds as well as various gases. Organic components consist of carbohydrates, proteins, fats and greases, oils, pesticides, phenols. Inorganic components consist of heavy metals, nitrogen, phosphorous, chlorides and other toxic compounds. In domestic wastewater, the organic and inorganic portion is approximately 50% respectively. Since wastewater contains a higher portion of dissolved solids than suspended, about 85% to 90% of total inorganic component is dissolved and about 55% to 60% of total organic component is also dissolved (Mara & Cairncross, 1989; Metcalf and Eddy, 1991; Ellis, 2004).
Biologically, wastewater contains various micro-organisms which include many pathogenic organisms, such as vibrio cholerae which generally originate from humans who are infected with disease or who are carriers of a particular disease (WHO, 2010).
3.2 Municipal wastewater treatment systems
It is estimated that by 2025 the majority of global population (over 5 billion) will live in the urban environments (UN, 2008; UN, 2013). Central to the urbanisation phenomena are the problems associated with the provision of municipal services such as fresh water resources, sanitation services and disposal of wastewater (Rose, 1999; WHO, 2010). The need to treat wastewater was recognized since the biblical days as a way of avoiding spread of communicable diseases and now is universally accepted as a norm (Amos, 2003). It is commonly accepted that the main objectives for treating wastewater are to reduce spread of communicable disease caused by sewage borne organisms, to prevent the pollution of surface and groundwater, to render it safe for reuse and to protect environmental integrity (Pescod and Arar, 1985; Mara, 1976; Alnos Easa and Ashraf Abou-Rayan, 2010).
Various wastewater treatment systems are available and are in use all over the world. Sewage treatment predominantly consists of physical (also called mechanical) and biological processes with chemical processes employed in a stage known as tertiary treatment (WHO, 2010). Conventional treatment systems are the most common methods of municipal wastewater treatment and comprise the following
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stages: preliminary treatment, primary sedimentation, biological treatment, secondary sedimentation and sludge treatment (Metcalf and Eddy, 1991; WHO, 2010).
Advanced wastewater treatment is the term applied to additional treatment that is needed to remove suspended and dissolved substances remaining after conventional secondary treatment. This may be accomplished using a variety of physical, chemical, or biological treatment processes to remove the targeted pollutants. Advanced treatment may be used to remove such things as colour, metals, organic chemicals, and nutrients (phosphorus and nitrogen) (The Green Lane, 2002; Metcalf and Eddy, 1991; World Bank, 2013).
Conventional treatment systems include activated sludge and trickling filter, whereas waste stabilisation ponds are examples of non-convectional system. Mechanised treatment systems are efficient in terms of their spatial requirements (0.5-1m2/Person Equivalent (PE) compared to natural treatment systems that are at 5-10m2 (PE). Conventional, aerobic, treatments results in maximum reductions in Biochemical Oxygen Demand (BOD) (Rose, 1999; World Bank, 2013). In Zimbabwe, waste stabilisation ponds, trickling filter systems and activated sludge are the main systems while septic tanks are in use in most low-density areas of major cities such as Harare, Bulawayo, Mutare and many other smaller towns.
3.3 Biological nutrient removal system (BNR)
A BNR plant employs the activated sludge system and is the most widely applied compact technology for sewage treatment (Mara, 1976; World Bank, 2013) because it has a high degree of operational flexibility. In activated sludge systems, microbes are held in suspension and it performs a wide range of biological processes on the wastewater as it passes through the aerated tank. The varying aerobic, anoxic and anaerobic conditions in the system encourage growth and activity of different microbes with specific action on the sewage (World Bank, 2013).
In the activated sludge treatment, natural processes are limited but at the same time intensified considerably. This is done by generating and maintaining a very high concentration of saprophytic bacteria and other micro-organisms in the aeration tank
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and by artificially increasing the oxygen supply through aeration (Misi, 2005). BNR plants cannot treat industrial wastewater influent effectively because industrial wastewater is generally toxic and deficient of nitrates and phosphates and this may result in the bulking of sludge (Mara, 1976; Lever, 2010).
3.4 Waste stabilisation ponds
Wastewater stabilisation pond technology is one of the most important natural methods for wastewater treatment. Wastewater stabilisation ponds are shallow man made basins consisting of single or several series of anaerobic, facultative or maturation ponds.
The primary treatment takes place in the anaerobic pond which is mainly designed to remove suspended solids and other soluble elements of organic matter. The secondary stage is done in the facultative pond and it removes the remaining BOD through the coordinated activity of algae and heterotrophic bacteria. The tertiary treatment takes place in the maturation pond removes the pathogens and nutrients (especially nitrogen).Waste stabilisation pond technology, is the most cost effective wastewater treatment for the removal of pathogenic micro-organisms because treatment is achieved through natural disinfection mechanisms and is well suited for tropical and sub-tropical countries as sunlight and temperature are key for the effectiveness of the processes (Mara 1976; Pescod and Arar 1985; Blumenthal, et
al. 2000, Lever, 2010)
Notable advantages of waste stabilisation ponds include simplicity in their design and construction, no need of electric power, little maintenance requirement, ability of ponds to absorb the hydraulic and organic shock loads and the capability to produce high quality effluent. The main disadvantages of waste stabilisation ponds are the large area requirement and the production of odour from the anaerobic ponds (Blumenthal, et al., 2000; Veenstra and Polprasert, 2000; Sperling, 2007; World Bank, 2013). The quality of the effluent is usually low and it requires to be disposed to land or pasture for tertiary treatment to improve its quality.
16 3.5 Nitrogen and phosphorus
Nitrogen together with phosphorus are essential to the growth of plants and as such are known as major nutrients (Metcalf and Eddy, 1991; Havens and Frazer, 2012). Plants and some micro-organisms readily absorb nitrates and ammonia ions from the soil. A high concentration of nitrogen may stimulate excessive growth and cause lodging, delayed crop maturity and poor crop quality (Amos, 2003, Havens and Frazer, 2012). However, most crops are not affected by nitrogen concentrations below 30 mg/l (DWAF, 1996a). Nhapi (2002) cites incidences where medium intensity irrigation with wastewater produced significantly higher yields than irrigation with fresh water supplemented with standard dozes of nitrogen, phosphorous and potassium. Plant uptake of nutrients accounts for up to 40% on nitrates applied depending on the crop type (Pescod, 1992; Majid Kermani, et al,. 2009).
Phosphorus is one of the essential plant nutrients and is frequently a limiting factor in vegetative productivity (Forth, 1984; Majid Kermani, et al., 2009). Applied phosphorous is either taken up by plants, incorporated into organic phosphorous or becomes weakly or strongly absorbed onto aluminium (Al), iron (Fe) and calcium surfaces depending on the pH (Veenstra and Polprasert, 2000). Continuous long-term application of phosphorous at levels exceeding crop requirements increases the potential of phosphorous loss through runoff and drainage water (Veenstra and Polprasert, 2000) leading to the eutrophication of surface water bodies. Long-term application results in the top 30 cm of the soil becoming saturated with phosphorous due to absorption, greater bioactivity and accumulation of organic matter (Veenstra and Polprasert, 2000; Rana, et al., 2010; Masona, et al., 2011).
3.6 pH
The pH of natural waters usually ranges between 6.0 and 8.5. Increased temperature or excess nutrients (phosphates and nitrates) may result in higher algal and plant growth, which may cause pH levels to increase, depending on the buffering capacity of the water. Photosynthesis of aquatic plants uses dissolved CO2, temporarily reducing the concentration of carbonic acid (H2CO3), a naturally
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occurring weak acid, and thus increasing the pH (Metcalf & Eddy, 1995; Githongo, 2010).
3.7 Metals in wastewater
A number of factors affect metal availability in soils. Bio-availabilities of metals are those metals that are in soil solution in a form that can be readily taken up by plants (Ncube, 2000; Rubio, et al., 2006; Masona, et al., 2011). High concentrations of Cadmium (Cd), Lead (Pb), Iron (Fe), Manganese (Mn), Aluminium (Al), Copper (Cu) and Nickel (Ni) pose a potential health hazard to humans and animals. Table 2 shows some selected Zimbabwe heavy metal guidelines for wastewater to be used for irrigation and may result in minimal health hazard. For the wastewater to be used for irrigation in Zimbabwe, the metal concentration values should be within the guideline range.
Pescod (1992) cited that copper, Zinc and Nickel are phototoxic and metals such as Cadmium (Cd), Silver (Ag) and Lead (Pb) are non-essential to the living being and have high toxic effects if they accumulate in the food web. Lead and cadmium metals are known to be cumulative and toxic and can affect animals, including human beings. For Zimbabwe, the maximum concentration for cadmium in irrigation water should be 0.011mg/l (Table 2). In plants, metals are known to interfere with the metabolic processes thereby affecting plant growth and crop yields (Madyiwa, et al., 2003).
Table 2: Zimbabwe Concentration of trace elements in wastewater suitable for irrigation
Element Long term conc. (mg/l) Short term conc. (mg/l)
Arsenic (As) 0.1 10 Boron (B) 0.75 2 Cadmium( Cd) 0.01 0.05 Chromium (Cr) 0.10 20 Copper (Cu) 0.20 5 Lead (Pb) 5 20 Nickel (Ni) 2.02 2 (Source: ZINWA 2000)
*The short-term concentration refers to the concentration which can be referred to as seasonal concentration.
18 3.8 Lead
Sources of lead in wastewater include batteries, domestic water distribution pipes, paint industries and petroleum (Amos, 2003; Kimbrough, 2009). The availability of lead in soils is related to the moisture content, soil pH, organic matter and the concentration of calcium and phosphates in wastewater (Ncube, 2000, Tjandraatmadja, et al., 2009).
Lead has a low phytotoxicity compared to other trace elements in wastewater. According to Metcalf and Eddy (1991), Ji-tao SI, et al., 2006 and Pinho and Bruno (2012) lead concentration tends to be higher in roots than in leaves, or in fruit parts of plants suggesting that translocation does not readily occur. Plants that display high bioaccumulation of lead include potatoes, lettuce and hay (Opeolu, et al., 2010; Uwah, et al., 2011).
Lead is not essential for plant growth, however plants take it up as Pb2+ when CEC, pH and available phosphorus decrease. Uncontrolled trace element input to the soil is undesirable as they are practically impossible to remove and may lead to toxicity to plants, adsorption by plants and they have subsequent adverse health impacts on humans or animals, and transport from soils to underground or surface water resulting in water unfit for use (ZINWA, 2000; Rattan, et al., 2001).
3.9 Materials and Methods
The study was conducted from January 2006 to February 2010 and involved collection of effluent, and analysing selected parameters of health, agriculture and environmental significance.
3.10 Study area
The study area is Luveve Gum Plantation area in Bulawayo , the second largest city of Zimbabwe. Zimbabwe is a tropical landlocked country in Africa and the country
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stretches between 150 301N to 220 301 S and lies between 250W and 330E with a population of about 13 million (Central Statistical Organisation (CSO), 2002).
Figure 1: Google map of Bulawayo showing wastewater treatment plants in relation to the Luveve gum plantation.
The city of Bulawayo is located in the drought prone, semiarid region of Zimbabwe. It is located on the central watershed of Umzingwane and Gwayi catchments with an average rainfall of 460 mm per annum. The city has an estimated population of 1 million. Water consumption in Bulawayo has been steadily rising over the past years with consumption now standing at about 43 Mm3 per annum (Taigbenu and Ncube, 2005).
Magwegwe ponds Farming
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3.11 Approaches in data collection and analysis
Parameters such as electrical conductivity, turbidity, temperature and pH were measured in the field for the effluent.
Table 3: Parameters measured and the methods used.
Sample Specific objective Parameter measured Number of samples analysed laboratory analysis of samples and the laboratories used
Effluent 1 and 3 pH, Electrical
conductivity, Temperature, Turbidity
150 Field testing kits
Effluent ( microbiology)
1 Total coli form
Faecal coli form
50 Zinwa
microbiology laboratory
Effluent 1 and 3 Total
phosphorus, Total Nitrogen, Magnesium, Potassium, Sodium 150 ALS and ZimLabs quality laboratory
Effluent 2 and 3 Lead and
Cadmium
50 Zimlabs quality
laboratory
3.12 Sampling procedure
Sampling sites were spatially selected on the farm. Five effluent sample points were selected, each with an identification number assigned to it. Sampling was performed every quarter for the period 2006 to February 2010.
21 3.13 Effluent sample collection
Two litre samples were collected as discrete samples at the five different sites into sample bottles (Figure 2) which were soaked overnight in dilute hydrochloric acid. Sample bottles were rinsed two times with sample before filling with sample following recommendations by APHA (1985). The samples were clearly marked with the date of collection and time, and put in a cooler box with some ice blocks for preservation. Effluent samples were taken to Zimlabs in Harare for analysis of nutrients and two heavy metals (Cadmium and Lead) as shown in table 3. Samples for metal analysis (Cadmium and Lead) were taken to the Geology Wet Chemistry Laboratory University of Zimbabwe for quality control of results. Faecal and Total coli form analysis was done within 6 hours of the last sample collection using the ELE Paqualab Kit (manufactured by E L E International Ltd).
Figure 2: Effluent sample collection (a): Effluent sample collection at site 1(b) Effluent sample collection at site3
3.14 Field based measurements
Turbidity was measured using a portable turbidity meter 350IR (manufactured by Hanna Ltd) which was first calibrated using turbidity standards. Conductivity and temperature were measured using TETRA CON 325, 340i conductivity meter (manufactured by VWR International) which was first calibrated using distilled water.
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The effluent pH was measured using Ecoscan pH 5/6 mV/pH meter (manufactured by Hanna Ltd) which was first calibrated by pH 4 and pH 7 buffer solutions.
The analysis of effluent at the water quality laboratory and Analytical Laboratory Services were done following recommendations by APHA (1989). Data analysis was carried out considering the potential wastewater effects on public health through irrigation and comparison was made to specific standards. The standards used in the data analysis are the South African Department of Water Affairs and Forestry (DWAF, 2006), Zimbabwe Water (Waste and effluent Disposal) Regulations, 2000, Food and Agriculture Organization, (FAO, 2006), United States Environmental Protection Agency (USEPA), 1983 and the World Health Organisation Guidelines (WHO, 2006).
3.15 Faecal and total coli form
Faecal and total coli forms were determined using ELE Paqualab Kit. For analysis of these parameters ringers solution was prepared in the water quality laboratory using NaCl-2.25 grammes, KCL- 0.105 grammes, CaCl2- 0.05 grammes, NaHCO3- 0.05 grammes and 1000 ml of distilled water. Sample bottles and apparatus for bacteriological examination were first cleaned and rinsed carefully giving a final rinse with distilled water and sterilized in an autoclave. A space of 2.5 cm was left in the collection bottles to facilitate mixing by shaking in preparation for examination and was placed in a cooler box with ice blocks before analysis within 6 hours of collection. Membrane Lauryl sulphate solution (broth) was used as the growth media to saturate the pad followed by filtration of sample. 20 ml of sample was diluted up to 10-5 in membrane lauryl sulphate solution before filtration. The filter paper was taken to the saturated pad in a perti-dish and then taken to an incubator for a minimum period of 16 hours. Total coliform was incubated at 370C and faecal coli form at 440C. Yellow/ pink or maroon colonies measuring 0.5 mm or greater were counted at the end of the incubation period and reported as cfu/100 ml.
Coli forms were calculated using the formula (4.1):
Cfu/100 ml = Number of coliform counted * 100 ml …………. (4.1) Number of millimetres filtered
23 3.16 Laboratory based analysis
Wastewater metals Analysis
The concentration of selected metals, Cadmium and Lead in the effluent was determined using the atomic absorption spectrophotometer (PU 9100 manufactured by Philips) in the wet chemistry laboratory of Zimlab, the Geology department and water quality Laboratory of the Soil science Department, University of the Zimbabwe. In the Analytical Laboratory Sciences, samples were digested using aqua rigia solution which has a ratio of 1:3 (HNO3 :HCL) after digestion, appropriate standards were prepared and concentration of metal was then read on atomic absorption spectrometer (Varian techtron spectra 50B 110 software) employing an air acetylene as fuel at wavelength of 217 nm for Pb and 228,8 nm for Cd.
Data analysis
Data collected from the analysis was entered into SPSS 10 for windows and analysed using descriptive statistics. Data from effluent analysis was entered into an Excel spread sheet. Descriptive and regression analysis statistics were performed using Excel 2003 version. One way ANOVA was carried out to compare the differences of means from various sampling sites, followed by multiple comparisons using the least significant test (Dunnett’s test) in which the level of significance was set at P=0.05 (one tail).
3.17 Results
3.17.1 Effluent characteristics of treated domestic wastewater used in Bulawayo
3.17.2 Effluent temperature
Effluent temperature was measured during the study period and a mean temperature value of 22.40C ±1.4 with a min of 200C and a max of 240C was obtained (see figure 5.2 below). Effluent temperature varied significantly among the years (P<0.05) with a slight decrease and was influenced by the hot climatic conditions of the area.
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Figure 3: Mean effluent temperature
3.17.3 Effluent pH
Effluent pH was measured and values obtained ranged from 6.89 to 8.6 and averaged 7.9 + 0.4 indicating a slightly neutral nature. Significant variations existed among the five sites for the period of study (P<0.05).
3.17.4 Effluent turbidity
The turbidity of the effluent ranged from 23.7 to 47.27 Nephelometric Turbidity Unit
(NTU) and a mean of 33.27 + 7.9 NTU. Variation of turbidity within the sampling
sites and sampling times was noted (P<0.05). The average value of 33.27 NTU for the effluent was higher than the recommended value of EAPSA which regulates that wastewater for irrigation should have a value around 2 NTU for unrestricted use while <2 NTU for restricted use.
16 17 18 19 20 21 22 23 24 25 2006 2007 2008 Years T emp 0 C Site 1 Site 2 Site 3 Site 4 Site 5
25 3.17.5 Electrical Conductivity (EC)
The average electrical conductivity of the wastewater was found to be 860.3 + 81.73 µS/cm with a range of 783.79 µS/cm - 986.86 µS/cm (see Figure 4). Analysis of variance indicated that there was a marked difference in the effluent electrical conductivity over the study period (P=0.01%).
Figure 4: Mean EC values against the minimum recommended standard
750 800 850 900 950 1000 1050 2006 2007 2008 EC measured Minimum std values µS/c m Years
26 3.18 Effluent chemical parameters
3.18.1 Effluent total nitrogen
Nitrogen in the effluent wastewater was observed to be in the range of 7 mg/l to 13 mg/l with a mean nitrogen concentration of 11.5 ± 4.4 mg/l.
3.18.2 Effluent total phosphorus
A variation of total phosphorus concentration in the effluent was observed and a mean of 13.5mg/l ± 14.9 was measured as shown in Figure 5. The effluent values ranged from 5 to 110 mg/l.
Figure 5: Concentration of total phosphorus in effluent
3.18.3 Cadmium concentration in the effluent
Cadmium concentration in the effluent was measured and a mean concentration of 0.027 mg/l ± 0.01 was observed. The Cadmium value ranged from 0.001 mg/l to 0.054 mg/l. The concentration of cadmium in the effluent is presented in figure 6.
0 5 10 15 20 25 30
Site 1 Site 2 Site 3 Site 4 Site 5
Sampling sites Average P values C o n c.mg /l
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Figure 6: Mean concentration of Cd in effluent STC: Short term recommended effluent concentration
LTC: Long term recommended effluent concentration
3.18.4 Effluent lead concentration
Total mean concentration of lead in the effluent was found to be 0.45 mg/l and the concentration of effluent ranged from 0.2 to 0.91 mg/l with a standard deviation of 0.186. Figure 7 shows the variation of lead concentration of effluent.
0.00 0.01 0.02 0.03 0.04 0.05 0.06 1 2 3 4 5 Samples sites Co n c . m g/ l Average Cd Conc. STC LTC
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Figure 7: Lead concentration in the effluent
3.19 Bacteriological quality of effluent
The value 5836 cfu/100ml for faecal coliform and 7291 cfu /100 ml for total coliform observed is above both the WHO and national recommended standards for irrigation. High total coliform counts are probably due to the inefficiency of the ponds in removing bacteria.
3.20 Discussion
3.20 Effluent temperature
Effluent temperature was measured during the study period and a mean temperature value was 22.40C ±1.4 with a min of 200C and a max of 240C was observed. Though temperature is generally not specified in most wastewater irrigation, water standards and guidelines, it has been reported to affect the metabolic rates, electrical conductivity and dissolved oxygen (Pescod, 1992; Mara, 2004). Effluent temperature has been shown to have some impact on the desorption of nutrients such as phosphorus. Studies by Mamo, et al., (2005), demonstrated that desorption was
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 1 2 3 4 5 sample sites Co n c . M g /l Pb Conc.
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higher at higher temperatures than compared to lower temperatures. Similarly an average temperature of 22.40C measured could probably influence desorption and leaching of nutrients. This phenomenon is highly linked to the fact that though cadmium and lead effluent levels were found to be high, their levels in soils and crops were found to be within the acceptable range for wastewater use. This observation to some extent confirms the observed influence of temperature by Mamo, et al., (2005) which in the case of Bulawayo, temperature could be postulated to have contributed to the desorption and leaching of heavy metals. However authors such as Gunawardana (2011) have also demonstrated that other factors also influence leaching and desorption and these were noted to be the nutrient soil levels, volume and rate of percolating water.
3.20.1 Effluent pH
A pH of 6.5 to 8.4 is desirable for effluent quality for irrigation according to the FAO (1985) guidelines and ZINWA guidelines of 2003.The observed pH that ranged from 6.89 to 8.6 with an average of 7.9 + 0.4 is within the desirable range. Tak, et al., 2012 and Feizi, (2001) observed that irrigation water with a pH outside the normal range may cause a nutritional imbalance. In the case of the effluent pH observed for the wastewater used in Bulawayo, the pH was within desirable range and this confirms its suitability for irrigation. According to the USEPA (1992), a low pH effluent of less than 6.5 promotes leaching of most heavy metals whereas a pH of greater than 11 destroys bacteria and while a neutral pH can temporarily inhibit movement of heavy metals through the soil. The average pH of 7.9 observed indicates that the wastewater is slightly alkaline. Alkalinity of wastewater has been demonstrated by Tak, et al., 2012 and Uwimana, et al., (2010) to affect mobility and uptake of heavy metals. The alkalinity of wastewater used in Bulawayo supports the findings of Uwimana, et al., (2010) and as such tests conducted on soils and plants in this study demonstrated that no significant levels of metals (Cd and Pb) were detected in the selected crops as the metals were immobilised.
30 3.20.2 Effluent turbidity
The high level of turbidity measured in this wastewater suggests that the channel that brings the wastewater to the site contributed significantly as it picked up sediments in the unlined canals to the field. In addition, the high turbidity observed can be attributed to growth of phytoplankton which has access to the nitrates and phosphates in the wastewater. The wastewater provides favourable conditions for the growth of phytoplankton as the temperature (22.40C) measured at the study site was ideal to support biochemical activities of aquatic species which is in agreement with observation by authors such as Alexander, et al., (2006) who reported a relation between temperature and turbidity.
Turbidity in the effluent was composed of organic and inorganic constituents derived from the households and also from the earth canal which is not lined at the study site (farm) (see figure 9). Higher turbidity levels, pose higher health risk to people as organic particulates harbour microorganisms. High turbid conditions have been reported to increase the possibility of waterborne diseases because particulate matter harbours micro-organisms and stimulates growth of bacteria thereby posing some health risk to the effluent users (Hoko, 2005; FAO, 2010). However at the study site, the household survey indicated that there was no significant incidence of diseases that could be linked directly to the wastewater use, thus indicating potential for use as long as safety precautions are taken.
3.20.3 Electrical conductivity
Electrical conductivity is widely used to indicate the total ionized constituents of water. It is directly related to the sum of the cations (or anions), as determined chemically and is closely correlated with the total salt concentration. The variance in EC values measured over the study period was expected because the conditions where the wastewater originates differed from day to day as it was influenced by the residents’ activities, such as saloons and backyard garages that contribute to the constituents of the wastewater.
